The present invention relates to high-voltage voltmeters in general and to long-range wireless phasing voltmeters in particular.
Three-phase high-voltage distribution and transmission lines consist of three energized conductors and a fourth, neutral or “ground” conductor. The three energized conductors each carry an electrical voltage that varies in magnitude at the same frequency but the phase of the voltage carried by each conductor is displaced by a phase angle of 120 degrees. The conductors carrying these three different-phased voltages are generally labeled as the A, B, C or 1, 2, 3 conductors, or equivalent depending on the utility, to tell them apart. In the simplest arrangement, the first phase, or reference phase, is arbitrarily designated to be 0 degrees, making the next phase 120 degrees displaced from the first and the last phase 240 degrees displaced from the first.
When two sets of high voltage distribution and transmission lines are to be connected, it is always, important to match the phases of each line. In total, there are six possible ways to attach any two sets of three conductors. Each of these six different connections will result in a different outcome for the device being powered, an outcome that may sometimes be significant. Incorrectly-wired three-phase transformer banks, consisting of three individual transformers, for example, can produce phase angles between 0 and 360 degrees in 30 degree steps. Accordingly, phase identification is an important measurement for those who maintain high voltage distribution and transmission systems.
Unfortunately, individual phase identification may get lost in overhead distribution and transmission systems. In underground electrical systems, which may extend for many miles, the phase identification ascribed to individual conductors is not always correct. Unauthorized digging or trenching up of an underground electrical system, which is a common occurrence, may result in loss of phase identification. Also, natural disasters such as accidents, hurricanes, tornadoes, forest fires, high winds, snow, ice, earthquakes, floods, etc. may result in loss of phase identification in above-ground and even in underground transmission systems. Mapping, phase tagging and verification of system records, for both above-ground and underground electrical systems, requires accurate phase identification.
Determining the time varying voltages of two conductors is part of the measurement but when the conductors are far enough apart, the fact that they are separated introduced errors into the comparison of the two measurements. Eliminating, correcting or avoiding those errors is vital to determining the phase difference.
Measuring the phase difference between the voltages on electrical conductors is known. One system is disclosed in U.S. Pat. No. 6,642,700 issue to Slade et al and assigned to Avistar Inc. This system identifies phase angles of electrical conductors at remote locations by measuring the time delay between an external clock source and a zero crossing of the wave form. A time tag is associated with that time delay and transmitted over a full-duplex communications link between a field unit and a reference unit. At the reference unit, the phase angle is calculated and displayed. The Avistar system uses global positioning system (GPS) as its external clock for determining the time delay.
Another phase angle measurement system is described in U.S. Pat. Nos. 6,734,658 and 7,109,699, issued to the present inventor. In this prior art system, a signal, corrected for capacitive charging currents, is obtained by a master probe measuring the voltage on a conductor in the field. The signal from the master probe is compared to another signal transmitted wirelessly and in full duplex from supplemental probe that has measured a reference voltage. The phase difference is displayed by the master probe. This system compensates for the phase shift introduced when a signal is sent from one probe to the other. The voltage signal being transmitted is encoded onto a carrier wave by modulating that wave with the voltage information itself.
These two prior art systems thus use two different ways of obtaining signals that represent the voltages of the reference and field conductors for comparison. The Avistar system compares time tags of the field signal and the reference signal, where the time tag of each is the difference between a GPS time and the zero crossing time of the alternating voltage, to determine the phase difference between the two varying voltages.
The Bierer system compares the reference conductor voltage and the field conductor voltage directly but compensates for the phase shift of the transmitted reference voltage caused by distance to the master probe measuring the field conductor voltage.
There remains a need for high voltage phasing voltmeter that is accurate, easy to read and can be used when the high voltage distribution or transmission lines are separated by many miles.